1. Field of the Invention
The present invention relates to a load driving device. More specifically, the invention relates to a load driving device including an output transistor that controls power supply to a load.
2. Description of Related Art
In order to supply power from a power supply to a load such as a functional circuit or a power device, a lot of load driving devices are employed. This type of load driving device includes an output transistor which is connected between the power supply and the load and serves as a switch. The load driving device supplies the power to the load or shuts off the power, according to a conduction state of the output transistor. The load driving device may be referred to as a high-side switch. An example of such a load driving device is disclosed in Japanese Unexamined Patent Application Publication No. 2007-19812.
A circuit diagram of a load driving device 100 disclosed in Japanese Unexamined Patent Application Publication No. 2007-19812 will be shown in FIG. 17. As shown in FIG. 17, the load driving circuit 100 includes a driver circuit 102, a switching N-type MOSFET (T101), and a controlling N-type MOSFET (T102). The controlling N-type MOSFET (T102) is provided between a gate of the switching N-type MOSFET (T101) and the ground. Then, the controlling N-type MOSFET (T102) is turned on or off, according to a driving signal s2 output from the driver circuit 102. When the controlling N-type MOSFET (T102) is turned off, a driving signal s1 output from the driver circuit 102 outputs a high-level signal. The switching N-type MOSFET (T101) is thereby turned on, so that power is supplied to the load 101. On the other hand, when the controlling N-type MOSFET (T102) is turned on, the gate of the switching N-type MOSFET (T101) is grounded to the ground. Thus, the switching N-type MOSFET (T101) is controlled to be turned off, so that power supply to the load 101 is shut off.
The controlling N-type MOSFET (T102) has a parasitic diode D102. An anode of the parasitic diode D102 is connected to the ground side, and a cathode of the parasitic diode D102 is connected to the gate side of the switching N-type MOSFET (T101). Thus, even when a dc power supply 103 is reversely connected, voltage is applied to the gate of the switching N-type MOSFET (T101) from the power supply 103 due to presence of the diode D102. Accordingly, the switching N-type MOSFET (T101) is turned on. For this reason, reverse connection current will not flow through a parasitic diode D101 of the switching N-type MOSFET (T101). Heat generation of the parasitic diode D101 may be thereby prevented. It means that in the load driving device 100, even when the power supply 103 is reversely connected, the switching N-type MOSFET T101 and other circuit component may be prevented from being damaged.
For the switching N-type MOSFET used as the output transistor in the load driving device, an N-type MOS transistor having a vertical structure formed on an N-type semiconductor substrate is often employed so as to realize high current capability. Further, in order to reduce the mounting area of the load driving device, it is preferable that the controlling N-type MOSFET (T102) be formed on the same semiconductor substrate as the switching N-type MOSFET (T101).